From: Wilba on
Pete wrote:
> Chris Malcolm wrote:

>> Different kind of directional. When a system is to seek a target and
>> stop when it gets there there can be a problem of dithering or hunting
>> close to the target due to tiny movements over the threshold, possibly
>> due to noise. So some kind of delay between on-target and off-target
>> conditions is imposed, often by giving some kind of snap action over
>> the target, so that it has to run past the target to be triggered as
>> target reached, and then under-run it by some amount to be triggered
>> as off-target.
>>
>> It's a kind of snap action. In the past often implemented with
>> snap-over springs on a mechanical switch. Now often implemented by
>> control software.
>
> Thanks very much for your input to this thread: I read it all yesterday,
> looked up the references (plus many others) and tried to understand how AF
> works.
>
> I thought I was doing well until I started thinking about the distance
> from the exit pupil to the sensor/film plane. E.g. a long distance gives
> the "narrow angle of attack" required by sensors to reduce vignetting -
> that implies a wide baseline would be inappropriate in this case. I guess
> that means I've missed a fundamental point (as happens quite often).
>
> The reason for my great interest is attempting to understand an AF
> problem, under very obscure lighting, with a non-Canon system: f/1.4 and
> f/1.8 lenses refused to focus anywhere near the subject; f/2.8 and f/4
> lenses were always spot on. Tests shots made on each lens from wide-open
> to f/5.6 showed the problem did not vary with shooting aperture. I
> certainly don't want to bore everyone to death trying to understand it
> all. Indeed, I'm happy to have learnt which lenses to avoid under certain
> lighting.

That sounds like exactly what Chris has been talking about - a static focus
error due to the mismatch of the lens exit pupil and the AF sensor's
required exit pupil ("effective aperture"). I wonder what would happen if
you put on an external aperture which reduced the exit pupil to that
f/2.8-f/4 range.


From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:0368c61e$0$1282$c3e8da3(a)news.astraweb.com...
[]
> But you did say, "you will likely stop at a different position, always
> within that dead-band", which doesn't happen with this system (it always
> stops at the ends of the beep band).

So your system doesn't behave in the way which I would consider "likely".
Maybe the software is very clever and works out the absolute minimum
distance to move the focus to be /just/ within that dead band? Without
the design details it's difficult to be sure.

>> However, given that there is noise in any electronic or optical system,
>> and given that there will be tolerances in any mechanical system, I
>> would not be surprised if given a fixed starting point, the AF system
>> did not stop at /precisely/ the same finishing point every time.
>
> Correct. Even CDAF has shot-to-shot variation. PDAF is a touch more
> variable. But we're not talking about micrometric precision, just not
> focussing between the end points of the beep band. The beep test shots
> match, so it seems that PDAF triggers focus confirmation on the
> transition, on entering or leaving the beep band.

Consistent with moving the minimum possible amount - i.e. getting the
fastest AF.

Cheers,
David

From: Wilba on
David J Taylor wrote:
> Wilba wrote:

>> David J Taylor wrote:

>> But you did say, "you will likely stop at a different position, always
>> within that dead-band", which doesn't happen with this system (it always
>> stops at the ends of the beep band).
>
> So your system doesn't behave in the way which I would consider "likely".
> Maybe the software is very clever and works out the absolute minimum
> distance to move the focus to be /just/ within that dead band? Without
> the design details it's difficult to be sure.
>
>>> However, given that there is noise in any electronic or optical system,
>>> and given that there will be tolerances in any mechanical system, I
>>> would not be surprised if given a fixed starting point, the AF system
>>> did not stop at /precisely/ the same finishing point every time.
>>
>> Correct. Even CDAF has shot-to-shot variation. PDAF is a touch more
>> variable. But we're not talking about micrometric precision, just not
>> focussing between the end points of the beep band. The beep test shots
>> match, so it seems that PDAF triggers focus confirmation on the
>> transition, on entering or leaving the beep band.
>
> Consistent with moving the minimum possible amount - i.e. getting the
> fastest AF.

Except it does the exact opposite - it makes front-focus out of gross
back-focus.


From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:036a2577$0$1375$c3e8da3(a)news.astraweb.com...
[]
> Except it does the exact opposite - it makes front-focus out of gross
> back-focus.

Doesn't sound to be working that well, then!

Cheers,
David



From: Wilba on
David J Taylor wrote:
> Wilba wrote:
> []
>> Except it does the exact opposite - it makes front-focus out of gross
>> back-focus.
>
> Doesn't sound to be working that well, then!

Well slap my kneebones to the ground! That's exactly where we started a
month ago. :- )